This invention refers to an ocean bottom geophysical exploration instrument, especially to a data acquisition array for conducting a marine multiwave seismic survey.
The current modes of a data acquisition array or system for conducting a marine multiwave seismic survey are described below. One mode is to adopt a streamer cable system in which the analog signals are transmitted in the cable. It will be digitized, recorded and processed in the central station. The shortcomings of this mode are that the quality of acquisition data is bad because the analog signal transmission line is too long, the signals are easily influenced, and the signals and noise are transferred in a mixed and overlapping mode so that the SNR of the acquisition data is lower and even signals are submerged by noise. Another mode is to adopt a distribution ocean bottom cable data acquisition array that uses mostly 12-24 channel data acquisition, with digitized data transmission in the cable and recording in a central station. For the current ocean bottom cable in some countries, many geophones are tapped on the cable. The geophone taken as a sensor cannot be towed because it is bound to the cable. The cable is too cumbersome with low strength of extension. With continuous movement and change of measuring points, the cable must be drawn and released frequently. For the above reasons, the labor intensity is very great so that it adversely influences the production efficiency. At the same time, it will cause a great interference on signals because the analog signal wire connected with the sensor is still too long, so the SNR of the acquired data is not high. This array is also complicated to manufacture and both difficult and expensive to maintain.
A description of existing products in this field can be found in the specification and introduction of pictures on pages 6 and 7 under Ocean Bottom Cable in the products brochure entitled SYNTRON of American Syntron Inc.
The aim of this invention is to provide a high-accuracy, high SNR, portable and towable integrated ocean bottom four-component array for seismic data acquisition. This integrated array can collect or acquire four key components of seismic signals simultaneously in total, which components are: one vertical component (V) of a seismic (P) wave using a geophone, two horizontal components (SX) and (SY) of a seismic shear wave using geophones, and one acceleration component (H) using a hydrophone. The first three components of the four above-mentioned components which are collected by the array of this invention are velocity components which are sensed using geophones as the velocity sensors; and, the fourth component, the one sensed using a hydrophone, is the acceleration sensor. According to the requirement in ocean bottom cable operation, several integrated sections or units can be connected in series via submarine cable to form a multi-channel seismic data acquisition system.
To achieve the aim of the present invention, an integrated ocean bottom towed four-component array for seismic data acquisition mainly consists of a submarine cable having a cable joint and a hydrophone section, a four-channel electronic section, and a three-component geophone section assembled in a sealed housing, and connected with the submarine cable through the cable joint. It is characterized in that
The said submarine cable has an armored structure in which a power line and a data transmission line are assembled in its central part; packing cords surrounding an internal protective layer are filled up around the armored structure; inner layer steel wire ropes and outer layer steel wire ropes are installed as a cable reinforcement part covered by an external protective layer.
The said hydrophone comprises several round ceramic wafers on the acoustically transparent housing and an impedance converting transformer to form a real acceleration sensor.
The said electronic section for seismic data acquisition comprises a CPU board, a data transmission board, an acquisition board for channel 1-2, an acquisition board for channel 3-4, and a power board on the printed circuit stand, having its (V), (SX), (SY) and (H) signal input channels for data acquisition;
The said geophone section is three velocity sensors comprising the two horizontal component geophones, and the one vertical component geophone on the same gimbal mount, with a short wire connected between the geophones and the electronic section internally;
Each section is designed as a four-component receiving point and is connected to each other section via said submarine cable in series to form a multi-channel acquisition system.
In other words, the integrated array of the present invention mainly consists of a submarine cable, hydrophone, a four-channel electronic section for seismic data acquisition, a three-component geophone section, a sealed housing and a cable joint. The three-component geophone section, the hydrophone section, and the electronic section are assembled in a sealed housing. After the housing is connected with the submarine cable through the cable joint, it forms a complete integrated ocean bottom towed four-component array for seismic data acquisition, that is, the various electromechanical parts as described are integrated into one unit. The geophones in the three-component geophone section are three velocity geophones. They are assembled on the same double-shaft gimbal mount, which can realize a flexible 360xc2x0 rotation to adjust the geophones to achieve optimal receiving status. In the hydrophone, an acceleration sensor is used. A short wire is connected internally between the geophones and the electronic section in order to reduce the signal interference caused by the environment and to improve SNR of data. All sections or parts in the integrated arrays for seismic data acquisition are connected with a water-resistant cable without taps. The cable is one with an armored structure in which two inner and outer layers of steel wire ropes are used as a reinforced part to enhance the cable intensity of extension. On the cable, there are no bindings such as geophones and so on so that it is easy to accommodate an ocean bottom towing operation. Each integrated array is designed as a four-component receiving point A multi-channel acquisition system is formed after several integrated arrays are connected with submarine cables.
The electronic section for seismic data acquisition comprises a CPU board, a data transmission board, separate acquisition boards for channel 1-2 and channel 3-4, and a power board on the printed circuit stand. In order to improve SNR and measurement precision, a structure without a backboard is used, and all the printed boards are connected directly with connectors. When data from the four components are transferred in the electronic section for seismic data acquisition, the high-frequency interference in signals can be filtered out through the line filter. The signals will be amplified or not after being transferred to the preamplifier and then sent to the A/D converter for digitization. According to sample rate and channel series, the digitized data are treated by the microprocessor and then transferred to the data flow of the data transmission cable. Then, the treated data are transmitted through the data transmission cable to a ground central station for data recording. The recorded tapes or disks are provided as source data tapes or disks to the processing center for data processing so that the desired oil and gas structure information can be obtained from the marine seismic survey.
This invention has the following advantages.
As there are only four channels for data acquisition in each electronic section, the dimensions of the integrated electronic section can be reduced greatly with a reduced length of the submarine cable connecting them. As the connecting wire is an internal short connection between the geophones as a sensor and the acquisition channel of the electronic section, and all the parts are sealed in an identical housing, the structure of the present invention can avoid the usual interference caused by the other objects on the analog signal line. As a result, interference caused by the environment can be reduced, and cross talk between channels can be avoided and, therefore, the SNR of the signals is improved. Three component geophones are installed on the same double-shaft gimbal mount, which can realize a 360xc2x0 free rotation to adjust the geophones to the optimal receiving status. As there are no sensor taps and also no bindings such as geophones and the like on the cable, it can be realized that the cable connection between all the electronic sections in a multi-channel acquisition system is sealed and connected with each other for use under water. The cable used can also be light-weight with a small diameter and is relatively less expensive than larger, heavy-duty cable. The production cost of the total integrated array or the acquisition system can be reduced correspondingly. This system can be easily and freely towed on a flat ocean sand bottom. Therefore labor intensity will be reduced when the present system is used, and the system can be easily maintained.